We describe a new approach for the controlled fabrication of ultrathin poly(epsilon-caprolactone) (PCL) electrospun nanofibers, with diameters ranging from 150 to 400 nm, from a solvent system based on a mixture of acetic acid and formic acid. We demonstrated for the first time the possibility of tuning the diameter and morphology of the nanofibers by the in situ modification of the molecular weight of the polymer, a consequence of the hydrolytic degradation to which the polyester is subjected in aqueous acidic media. A study of the PCL degradation kinetics enabled precise adjustments of polymer molecular weight and thus of the solution viscosity. Hence, regimes and boundaries of PCL electrospinning in this solvent system could be determined, ranging from electrospraying to continuous fiber electrospinning. (C) 2013 Elsevier Ltd. All rights reserved.